As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2010.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
Heat-treatable AlMgSi alloys of the AA6000-type series are being used widely for automotive body parts in which, besides good formability of the aluminium sheet, the strength after undergoing a paint bake cycle plays an important role. The requirements imposed on the aluminium sheet used for the automotive body parts include good formability, a low and stable yield point, a high surface quality including amongst others that no stretcher strains are present following shaping or deforming the sheet into a bodywork panel. Conventionally, such aluminium sheet is manufactured by casting an ingot, homogenisation and/or pre-heating, hot rolling, cold rolling to final thickness, followed by solution heat treatment and quenching from solution heat treatment temperature so as to cool to below 150° C. using a high cooling rate. Following the cooling the aluminium sheet is then aged, in particular by natural ageing, to obtain the desired level of properties. For use in the automotive industry the aluminium sheet, following shaping by for example pressing or deep drawing, is provided with one or more paint layers. Such a paint layer is fast cured by maintaining it for some time at elevated temperature. Such a treatment is in the art designated by the terms “paint-bake” or “paint-baking”, and going through such a treatment is designated in the art by the term “paint-bake cycle”. A typical paint-bake cycle comprises of one or more sequential short heat treatment in the range of up to 200° C. for a period of 10 to less than 40 minutes, and typically of less than 30 minutes. In dependence of the OEM such a paint bake cycle may comprise of 2 to 5 sequential steps and includes drying steps.
Artificial ageing of the AA6000-series aluminium sheet and curing of the paint layer usually largely coincide. The trend among automotive manufactures is to reduce the time and temperature of the paint-bake, for example to 15 to 30 minutes at about 160° C. to 190° C., due to the need for energy saving. Several proposals have been made for a preliminary ageing treatment or pre-age treatment to improve the properties of the sheet following the paint-bake. Such a pre-age treatment is carried out after quenching from solution heat treatment and before any substantial age hardening has taken place, and whereby the pre-age treatment is carried out prior to a forming operation (e.g. stamping, deep drawing) in order to increase the so-called paint bake response, the latter being a significant strengthening of the alloy sheet during the automotive paint bake cycle. The application of a pre-age treatment enhances the kinetics of precipitation and decreases the precipitate size. Examples of such pre-age treatments are disclosed in the patent documents WO-95/14113, WO-96/07768, WO-97/022724, and U.S. Pat. No. 3,135,633.
However, there is an ongoing trend among automotive manufactures (OEM's) to further reduce the temperatures used in the paint-bake cycle resulting in that the currently used 6000-series alloys do not achieve the required strength levels after the paint-bake cycle, even not when the known pre-ageing treatments have been applied. Hence, the methods of manufacturing automotive body parts needs to be further altered in order to achieve the desired strength levels in the aluminium alloy product after a paint-bake cycle.